Randomly mid-match a Discord Screen pops up inside of the Overwatch app, not as its own window, and there’s seems to be no way of closing or minimizing it without closing the Overwatch app entirely. The Discord Screen shows an error stating: " Loading chunk 77856 failed."
Although this screen covers the entire Overwatch app, it doesn’t seem to be a connection issue, as I can still shot and use my abilities, and can still hear the rest of the match.
That’s called the Discord overlay, which is not part of Blizzard’s game. You need to turn it off inside the Discord client.